In a dynamic web application, the session is crucial to hold the information of current logged in user identity/data. So someone without authentication cannot have access to some Page or any ActionResult , to implement this kind of functionality, we need to check session exists (is not null ) in every action which required authentication. So, the general method is as follows: [HttpGet] public ActionResult Home() { if(Session["ID"] == null) return RedirectToAction("Login","Home"); } We have to check the above 2 statements each time and in each ActionResult , but it may cause 2 problems. Repeat Things : As per the good programming stranded, we don't have to repeat the things. Create a module of common code and access it multiple times/repeatedly Code missing : We have to write code multiple times so it might happen some time we forget to write code in some method or we missed it. How To Avoid? The ASP.NET MVC provides...